1. Field of the Invention
The present invention relates to an auto-focusing apparatus used for an electronic still camera, a video camera, a photographic camera, a telescope, and the like.
2. Description of the Related Art
As a conventional auto-focusing apparatus, an apparatus disclosed in "Automatic Focus Adjustment of Hill-Climbing Servo Scheme in TV Camera", NHK Technical Research, Vol. 17, No. 1, Serial No. 86, 1965, pp. 21-37 is known. In the hill-climbing servo scheme, a predetermined frequency component is extracted from a video signal obtained by a TV camera, and a photographing optical system is moved to a position where the amount of the image signal of the extracted frequency component exhibits the maximum, thereby performing focus adjustment. The hill-climbing scheme requires no special optical component for focus adjustment and hence allows a reduction in size of the apparatus. In addition, a high-precision focusing operation can be performed regardless of the pattern of an object to be photographed.
In the hill-climbing scheme, however, an optical system hunts near an in-focus point, resulting in a low focusing speed. In order to solve this problem, the present applicant has already filed a patent application associated with an auto-focusing apparatus for calculating an in-focus point by interpolation processing based on image signal values at several points near the in-focus point (U.S. Pat. application Ser. No. 485,646, filed Feb. 27, 1990).
This auto-focusing apparatus will be briefly described below with reference to FIGS. 1 to 3. As shown in FIG. 1, in the auto-focusing apparatus, an optical image of an object to be photographed (not shown) obtained by a photographing optical system 1 is incident on a light-receiving surface of a charge coupled device (to be referred to as a CCD hereinafter) 2 as a two-dimensional imaging device. It is to be noted that the two-dimensional imaging device is not limited to a CCD. For example, a metal-oxide semiconductor (to be referred to as an MOS hereinafter) may be used. Furthermore, a solid-state imaging device need not necessarily used, but an imaging tube or the like may be used. Charges generated by the irradiation of the object image light onto the CCD 2 are stored in the CCD 2. The charges are then read from the CCD 2 as an image signal every predetermined time interval in response to read signals supplied from a driver 3. The image signal is input to a band-pass filter (to be referred to as a BPF hereinafter) 5 and a pre-metering circuit 6 through an amplifier 4.
The pre-metering circuit 6 determines a charge storage time, of the CCD 2, which allows proper exposure. The resultant time signal is supplied to a microprocessor 7. The microprocessor 7 supplies a command signal to the driver 3 on the basis of the time signal, thus controlling the charge storage time for proper exposure.
Meanwhile, the image signal component of a specific frequency band is extracted from the image signal input to the BPF 5 and is supplied to a gate 8. The gate 8 extracts only a signal component associated with a target in-focus region from the image signal of one frame and supplies it to a detector 9. The detector 9 is formed of, e.g., a square detector for detecting the square sum of signals and detects the amplitude of the signal component associated with the target in-focus region and supplies it to a digital integrator 11 through an A/D converter 10.
The digital integrator 11 is formed of an adder 12 and a latch 13. Signal values input to the digital integrator 11 are sequentially added together to output a specific frequency component value (to be referred to as a focus signal value hereinafter). This focus signal value corresponds to a degree of focusing. A focus signal f(x) is generated from a plurality of focus signal values at each position of the optical system 1. That is, x of the focus signal f(x) represents the position of the photographing optical system 1 in the direction of the optical axis. The focus signal f(x) is supplied to the microprocessor 7. The microprocessor 7 stores the focus signal f(x) in a memory 14. The microprocessor 7 executes an in-focus point detecting operation by using the focus signal f(x) to generate a driving control signal. The driving control signal is supplied to a motor driving circuit 15. The motor driving circuit 15 controls a pulse motor 16 for moving the photographing optical system 1 in the direction of the optical axis so as to perform focus adjustment.
An operation of the conventional apparatus having the above-described arrangement will be described below. It is assumed that the interlace scheme is employed for imaging the object, and image signals are obtained as field image signals. Field image signals are read from the CCD 2 every field period. The pre-metering circuit 6 determines a proper charge storage time from the first field image signal and supplies the charge storage time data to the microprocessor 7. Subsequently, the microprocessor 7 controls the driver 3 in accordance with the determined storage time, thus supplying read signals to the CCD 2 in a predetermined cycle. Meanwhile, the motor driving circuit 15 causes the pulse motor 16 to drive the photographing optical system in a given direction. In this case, the moving speed is constant. At this time, the image signals read from the CCD 2 are formed into the focus signal f(x) through the BPF 5, the gate 8, the detector 9, the A/D converter 10, and the digital integrator 11. In this manner, the focus signals f(x) are obtained at predetermined time intervals in the process of the movement of the photographing optical system 1, that is, the focus signal f(x) is obtained every time the optical system 1 is moved by a predetermined distance. The microprocessor 7 drives the photographing optical system 1 in the direction to increase the level of the focus signal f(x).
FIG. 2 shows the focus signal f(x) in a form of a combination of discrete signal values obtained in this manner. FIG. 3 is an enlarged view of a portion Q near the maximum value of the focus signal curve shown in FIG. 2. The microprocessor 7 calculates a position .alpha. of the optical system 1 at which a peak value Px (=f(.alpha.)) of the focus signal is obtained by interpolation processing based on the following formulas using a maximum point P.sub.1 (=f(x.sub.m)) and two points P.sub.0 (=f(x.sub.m-1)) and P.sub.2 (=f(x.sub.m+1)) on both sides thereof of the sample values of the focus signals: EQU If P.sub.0 .ltoreq.P.sub.2, EQU .alpha.=x.sub.m -(.DELTA.x/2)(P.sub.0 -P.sub.2)/(P.sub.1 -P.sub.2) (1a) EQU If P.sub.0 &lt;P.sub.2, EQU .alpha.=x.sub.m +(.DELTA.x/2)(P.sub.2 -P.sub.0)/(P.sub.1 -P.sub.0) (1b)
where x.sub.m is the position of the optical system 1 at which the maximum value P.sub.1 is obtained, and .DELTA.x is the distance (sampling interval) at which the optical system 1 moves for a predetermined period of time.
When the photographing optical system 1 is moved to the in-focus point .alpha. calculated by the interpolation processing based on equation (1a) or (1b), focus adjustment is completed.
In this method, if the sampling interval .DELTA.x is increased, the number of times of sampling (detection) of the focus signals f(x) can be reduced to increase the focusing speed. However, since an in-focus position is detected by interpolation processing, if the sampling interval .DELTA.x is increased, an error due to interpolation is increased, resulting in poor focusing precision. In contrast to this, in this interpolation scheme, if the sampling interval .DELTA.x is too small, the detection of an in-focus position tends to be influenced by noise. This also adversely affects focusing precision. As described above, in the conventional method, a sampling interval which minimizes an error cannot be determined, and it is difficult to balance focusing precision and speed.